I have owned the original 8x for several years, and after purchasing a new battery for it (the battery it originally came with might have been defective or old), liked it a lot. I then purchased an 8ax about a year ago. It is slightly larger in length and diameter than the 8x, and has more of a tactical look, but is pretty much the same (same battery, same lamp, same light output, etc.). Also, the 8ax beam pattern is different (larger, more "blended" hotspot than the 8x). I then bought the 8nx. Similar in almost all respects to the 8ax. I did return to the 8x, however; the 8nx and 8ax are kept as backups.
I did drop the 8ax at one point, and the pyrex lens cracked ... I called SF CS, and they promptly shipped me a new bezel/lens. /ubbthreads/images/graemlins/thumbsup.gif
In terms of duty light use, I couldn't tell you a big difference between the ax and the nx. Moneywise, the nx is considerably cheaper, and just as durable as the ax.
That being said, although I really like my 8x, and the 8ax and 8nx are decent lights, did you consider possibly getting a SL TL3led? Output of 85 lumens is just a little less output than the SF 8 series at 105 lumens. No need to pay $25+ for replacement lamps, and estimated runtime is 4 hours at high brightness, with 50 hours at decreasing brightness. It uses CR123 batteries, but with SF brand batteries, a change of batteries would run about $4.50. Cost range for the SL TL3led is $105-120, which is about what the 8nx would cost, and less than the 8ax. I ordered one last week, and am anxiously awaiting its arrival. Just thought you might want to explore your options in a midsized duty light. /ubbthreads/images/graemlins/smile.gif With the money you're looking to spend, you do have several to look at.
Good luck /ubbthreads/images/graemlins/grin.gif
P.S. - In all things Surefire, you might want to P.M. Size15s, our resident SF go to guy. /ubbthreads/images/graemlins/smile.gif